fix(mcp): accept schema-valid empty strings in clientInfo identity fields

`isImplementation` reused `stringField`, which requires a non-empty string, for
the required `name` and `version`. The 2026-07-28 schema declares both as plain
`string` with no minimum length, so `{name: "", version: ""}` is a conforming
`Implementation` and was being answered -32602. Required now means present and
a string.

`Icon.src` gets the same treatment. Its `format: uri` annotation is not
something this server enforces — any other non-URI string is accepted — so
rejecting the empty one alone was arbitrary rather than stricter.

Adds positive regressions at both layers for empty `name`/`version` and an
empty `Icon.src`, alongside the existing malformed cases, so the validator is
pinned against over-rejection as well as under-rejection.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TKWswqMZ6Z9UAQDAda83Xz

Author: claude

src/mcp/__tests__/protocol-era.test.ts

@@ -113,6 +113,16 @@ test('a supplied clientInfo must be an Implementation, but omitting it is fine',
   const valid = { name: 'c', version: '1' };
   assert.equal(resolveProtocolEra('tools/list', modernMeta()), 'modern');
   assert.equal(resolveProtocolEra('tools/list', withClientInfo(valid)), 'modern');
+  // Required means present and a string, not non-empty: the schema sets no minimum
+  // length on `name`/`version`, nor on `Icon.src`.
+  assert.equal(
+    resolveProtocolEra('tools/list', withClientInfo({ name: '', version: '' })),
+    'modern',
+  );
+  assert.equal(
+    resolveProtocolEra('tools/list', withClientInfo({ ...valid, icons: [{ src: '' }] })),
+    'modern',
+  );
   // Every recognized optional field, plus an unknown extension key, stays acceptable.
   assert.equal(
     resolveProtocolEra(

src/mcp/__tests__/router.test.ts

@@ -249,6 +249,21 @@ test('a supplied clientInfo must be a valid Implementation', async () => {
   });
   assert.ok(omitted && 'result' in omitted);
 
+  // `name` and `version` are required `string` with no minimum length, so an empty
+  // string is a conforming Implementation and must not be rejected.
+  const emptyStrings = await handleMcpMessage({
+    jsonrpc: '2.0',
+    id: 'empty-client-info',
+    method: 'tools/list',
+    params: {
+      _meta: {
+        ...MODERN_META,
+        'io.modelcontextprotocol/clientInfo': { name: '', version: '' },
+      },
+    },
+  });
+  assert.ok(emptyStrings && 'result' in emptyStrings);
+
   // A fully populated clientInfo, plus an extension key, must still be served:
   // validation must not harden into rejecting what the spec allows.
   const rich = await handleMcpMessage({

src/mcp/protocol-era.ts

@@ -202,11 +202,14 @@ function isRecord(value: unknown): value is Record<string, unknown> {
  * Whether a value is a valid `Implementation`: `name` and `version` are required, and
  * every other recognized field is type-checked when present. Unrecognized keys pass —
  * `_meta` payloads carry extension fields, and rejecting those would reject the future.
+ *
+ * Required means *present and a string*, not non-empty: the schema declares plain
+ * `string` with no minimum length, so `{name: "", version: ""}` is a conforming
+ * `Implementation` and refusing it would reject a client the spec allows.
  */
 function isImplementation(value: unknown): boolean {
   if (!isRecord(value)) return false;
-  if (stringField(value, 'name') === undefined) return false;
-  if (stringField(value, 'version') === undefined) return false;
+  if (!isString(value.name) || !isString(value.version)) return false;
   return (
     isOptional(value.title, isString) &&
     isOptional(value.description, isString) &&
@@ -215,10 +218,15 @@ function isImplementation(value: unknown): boolean {
   );
 }
 
-/** An `Icon` requires `src`; the rest are optional but typed when supplied. */
+/**
+ * An `Icon` requires `src`; the rest are optional but typed when supplied. `src` is
+ * checked as a string for the same reason as `name`/`version` — its `format: uri`
+ * annotation is not something this server enforces, so rejecting `""` while accepting
+ * any other non-URI string would be arbitrary.
+ */
 function isIcon(value: unknown): boolean {
   if (!isRecord(value)) return false;
-  if (stringField(value, 'src') === undefined) return false;
+  if (!isString(value.src)) return false;
   return (
     isOptional(value.mimeType, isString) &&
     isOptional(value.sizes, isStringArray) &&